Abnormalities of MC placenta
sharing have received considerably less attention by scientists
than the connecting blood vessels, but are just as important.
Monochorionic twin placental asymmetry has been variously
called 'unequal sharing of venous return zones,' 'unequal
allocation of parenchyma', or 'discordant vascular perfusion
zones,' but a precise definition is lacking. The percentage
of each twin's portion of the MC placenta cannot be directly
determined by ultrasound scanning during pregnancy, but
can be estimated during postpartum placental examination
by noting the location of the vascular equator in the shared
placenta.

The cause of MC placental
asymmetry is also unknown, but the MC twin embryo (blastocyst)
seems to have a problem when it implants into the lining
of the mother's womb (see Figure 5). Because there are two
different areas of cells in the blastocyst that will become
babies, and have to develop their own healthy placenta,
events that should lead to a normal placenta in a single
baby cannot occur effectively when there are two future
babies in the blastocyst.

The portions or shares
of the MC twin placenta in TTTS are often unequal with the
donor's typically the smaller. The threshold for significantly
abnormal sharing (e.g., 60:40, 70:30, 80:20, 90:10, etc.)
and placental insufficiency, which might lead to one twin
being smaller than its co-twin or even jeopardize its normal
growth and survival in the womb, may vary in each case and
depend on the month of pregnancy, and type and number of
connecting vessels. One study showed that if the placental
asymmetry was 60:40 or more, a significant difference in
twin birth weight could be expected in TTTS cases.

In cases without connecting
vessels, unequal sharing is an important cause of size and
growth differences in identical MC twins. The presence of
connecting blood vessels can place a MC twin with an adequate
placenta share at risk for abnormal events that may occur
in the twin with placental insufficiency. Conversely, the
connections may help a twin with a small share by supplementing
nutrients that would otherwise be deficient. The clinical
consequences of MC asymmetry depend on its degree, the type,
direction and number of vascular anastomoses, and the gestational
age (see Figure 7). Theoretically, there must be a placental
share (perhaps less than 20%) that is incompatible with
continued intrauterine survival of one twin.

In addition to the sharing
differences, the asymmetric MC portions may differ qualitatively
in placental circulation relative to the umbilical cord
insertions, placental surface blood vessel pattern and the
way blood flows deep within the substance of the placenta.
Abnormal umbilical cord insertions and single umbilical
cord artery (normally there are two) are more common in
twins than single babies and are associated with smaller
placentas. Not surprisingly there is a relationship of velamentous
cord insertion (see Figures 3 and 4) to MC twins with TTTS
(65% in TTTS versus 20% in MC twins without TTTS), and cases
with velamentous cords have worse outcomes and earlier delivery
despite attempts at treatment. Velamentous cord insertion
and placental asymmetry were linked by one investigator
who found a moderate to marked MC placental asymmetry and
a small share for the twin with a velamentous insertion,
particularly if the co-twin had central cord insertion (see
Figure 4). Single umbilical artery occurs three to four
times more frequently in twins, and when present in only
one twin it is usually the smaller. Finally, when one looks
at the twins' placental portions in TTTS cases with a microscope,
the donor portions have blood vessels that are fewer in
number, compared to the recipient who has more dilated and
congested blood vessels.

Sadly, some MC
twins may have shares of the common placenta that are unable
to sustain their lives in the womb to a point where they
can survive if delivered. In such cases additional placental
tissue cannot be created. When this twin passes away, the
other twin is at risk for death or birth defects because
of the connecting vessels. The only therapy that can remove
these risks to the other twin (with the larger or normal
placental share) is laser occlusion of all the vessels because
it will 'disconnect' the MC twins.